The present invention is directed to an aqueous process for reducing particle size of organic pigments by milling the crude pigment in the presence of styrene copolymer dispersants.
It is well known in the art that organic pigments, such as quinacridones, as synthesized, are generally unsuitable for use as pigments and must be further processed to develop the requisite pigmentary properties such as particle size, particle shape, polymorphic phase, and tinctorial strength.
In order to obtain the color properties required for a particular application, the pigment crude must be converted to a pigmentary grade with a proper tint strength, transparency or opacity for a particular application. The effectiveness of a given pigment type in imparting color is dependent upon it's particle size in dispersion. Thus, color strength, transparency and opacity are all properties that are highly dependant on particle size. Consequently, crude organic pigments undergo one or more finishing or conditioning steps that require particle size reduction. See, for example R. B. McKay, “Control of the Application Performance of Classical Organic Pigments” in JOCCA, 89–93.
For example, quinacridone can be synthesized by a peroxide process, as described in U.S. Pat. No. 5,840,901. The obtained pigment is called a pigment crude because the pigment does not have the color property needed for various applications. Further size reduction is necessary. There are many traditional ways of achieving proper pigment particle size including dry milling processes, such as those published in U.S. Pat. Nos. 2,402,167, 3,030,370 and 5,084,100. These references disclose pigment particle size reduction by milling dry pigment powder in the presence of large amounts of inorganic salt and metal nails and/or balls. In this process, pigment crude is first milled to an almost amorphous phase, followed by a re-growth process in order to achieve the proper crystal size. This milling of pigment crude followed by the re-crystallization makes this finishing process complicated and costly. Another traditional pigment finishing process is acid drowning, as described in, for example, U.S. Pat. Nos. 3,326,918, 3,607,336 and 4,455,173. In this process, crude organic pigment is dissolved in a strong acid such as sulphuric acid, often in the presence of polar organic solvent. Then the pigment is drowned out and re-crystallized into an appropriate particle size designed for a particular application. In both processes, the initially synthesized pigment crystal is more or less destroyed then rebuilt, adding significant cost to the final product. Further, the dry milling process requires the presence of inorganic salts and re-crystallization in strong acid, generating significant environmental waste. The dissolution process similarly generates large quantities of waste acid.
Milling methods are known for improving properties of various organic pigments. Some examples are in U.S. Pat. Nos. 6,210,474, 4,597,794, 5,231,131 and 5, 530,043. However, the milled pigment in these patents is not the crude organic pigment but a finished pigment. Additionally, the dispersing milling agent is not a styrene copolymer.
U.S. Pat. No. 6,410,619 describes a method for conditioning of crude organic pigments using pure acrylic copolymer dispersants. Styrene copolymer is disclosed in EP 496 149 in making aqueous emulsions for use in graphics. JP 55089366 and U.S. Pat. No. 4,293,475 disclose styrene copolymers for making pigment dispersions. Patents WO9905575, EP 636,942 and U.S. Pat. No. 5,432,036 disclose styrenic polymer based resins for making toner compositions for dry electro-photoconductive imaging. U.S. Pat. No. 2,816,115 describes the use of a hydrolysed styrene-maleic anhydride copolymer for dispersing crude phthalocyanine pigments. U.S. Pat. No. 6,056,814 describes the use of a styrene copolymer for dry milling a crude organic pigment.
There is a need for a process that reduces particle size but does not destroy the crystal structure of the pigment and that can be used for both particle size reduction of beta and gamma crystal phases. There is also a need for an environmentally sound process that does not produce large amounts of waste in the milling or recrystallization. Additionally there is a need for a process that allows the reduction in particle size at high pigment loadings thus increasing the manufacturing through-put. Further, there is need for a process that does not require the crude pigment to be dried, a considerable energy cost savings, before reducing particle size. Accordingly, the process of the instant invention has all of the above advantages.
None of the above references disclose the use of a styrene copolymer dispersant as an aqueous milling agent for the purposes of reducing particle size to the nanometer range to improve the optical properties of a crude organic pigment.